How can I sign a smart contract transaction data and validate it properly?

Hello folks,

How are you today, I hope you all are doing well!

I am struggling with something I want to achieve with smart contracts. First of all, I am very new to this area so please consider me as a super noob.

Since all the smart contracts are publicly available to anyone, how is it possible to send a signed data by user X. For example, I want to create a smart contract which can be transact by any user (User X). The amount of the required ETH should be determined by my own web service but the transaction will be executed by user x. The smart contract will not hold data or do any check regarding the ETH amount. But within the smart contract I need to validate that the sent data is not edited by the user x. Does anyone know how I can achieve this scenario?

So imagine there is a marketplace with users can set their own ETH amount. Once a buyers wants to buy the regarding object, a transaction data (with the required ETH amount) will be generated by my web service. This generated data will be sent to the smart contract with the required ETH amount. After it's validated, the transaction should pass.

Thanks in advance!